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How do I choose the right timber for deck joists?

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How do I choose the right timber for deck joists?


I'm a newbie attempting a deck. Above are my basic plans and rough costings

I'm a bit overwhelmed at the choice of timber for the joists.

I plan to use Tuffblocks for the foundations (the 'dots' in my diagram). The deck is 3400 x 4400 in order to hold a 3000 x 4000 Mimosa Lava Portsea gazebo fixed on the top. So I needed to consider 2 things - hiding the Tuffblocks (they are 300mm diameter - I've drawn the 4 corner ones in as circles) AND making sure the Gazebo poles sit over the corner foundations. So the deck extends a total of 200mm out each side (with noggins to stablise)

S0 I've sourced all materials - but what timber should I use? It sits in the tuffblocks, has to be at least 35mm depth. Ideally I'd rather make them 3400mm long (so not buy 3000mm lengths and have to join them)

The deck will be made above an existing brick paving area with hte joists sitting 90mm above ground.


Bunnings Team Member
Bunnings Team Member

Re: How do I choose the right timber for deck joists?

Welcome to the Bunnings Workshop community @womby36. It's great to have you join us, and thank you for sharing your question about how to choose the right timber for your deck.


Let's do some quick measurements. If you use 140 x 45mm Treated Pine Outdoor Timber Framing - 4.8m and add the thickness of the deck panel 19mm for Merbau decking plus your 90mm space, you'll have an overall measurement of 249mm. At that height, you will have to make a timber step in order for people to get onto the deck. The 140 x 45 comes in 3.6 meters giving you just a little bit more than you need. The offcuts can be used for the picture framing side of the decking frame.


However, if you use Treated Pine Outdoor Timber Framing 90 x 45mm plus the timber decking panel 19mm and the 90mm space, you will have an overall measurement of 199mm. The 90 x 45 comes in different lengths so it's important to check with your local store that they have the lengths you need. If not you can speak to your local store's Trade Desk and ask them if they can order you the lengths necessary for your deck. Since the 90 x 45 is not as thick as the 140 x 45 you will have to reconsider the number of  Tuffblocks you will use.


I wouldn't recommend going any smaller as far as timber size is concerned. Getting thinner-sized timber will produce any number of decking defects. Sponginess, bounciness, and timber twisting which is prone to happen with smaller sized timber. I suggest engaging the services of a deck builder or engineer to sign off on your plan. 


Let me tag our experienced members @tom_builds, @Brad, and @MGusto for their recommendations.  


If you need further assistance, please let us know.




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Experienced Contributor

Re: How do I choose the right timber for deck joists?

Hi @womby36 and thanks for the tag @EricL .


Sorry I'm a bit late to reply, you've done a great job on your plans there @womby36 !

With the number of tuff blocks in your plan I think you will be fine with 90x45 as your joist spans is within the appropriats range . You should be able to get your hands on 3.6m lengths - if not at a large local bunnings then through the trade/special orders teams.


Keep us updated as you progress with your project!

Best of luck,




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